1. Field of the Invention
The present invention relates to a projection exposure apparatus used for the transfer of a circuit pattern of an integrated semiconductor device or the like, or of a pattern of a liquid crystal display device.
2. Related Background Art
For the formation of a circuit pattern of a semiconductor device or the like, there is generally employed a process called photolithography. This process usually employs a method of transferring a reticle (or mask) pattern onto a substrate such as a semiconductor wafer. Said substrate is coated with photosensitive photoresist, to which the circuit pattern is transferred according to an irradiating optical image, or the form of transparent portions of the reticle pattern. In a projection exposure apparatus, the circuit pattern which is formed on the reticle and is to be transferred, is focused on said substrate (wafer) through a projection optical system.
In an illuminating optical system, for illuminating the reticle, includes an optical integrator such as a fly's eye lens or optical fibers, thereby obtaining a uniform intensity distribution in the light illuminating the reticle. In order to attain optimum uniformity, in the use of a fly's eye lens, the focal plane of the fly's eye lens at the reticle side and the pattern bearing face of the reticle are substantially correlated by the relationship of a Fourier transformation, and the focal plane at the reticle side and the focal plane at the light source side are also correlated by a Fourier transformation. Consequently the pattern bearing face of the reticle and the focal plane of the fly's eye lens at the light source side (more precisely the focal planes of individual lenses of the fly's eye lens at the light source side) are in an imaging (conjugate) relationship. Therefore, the intensity on the reticle is averaged by the addition (superposition) of the illuminating lights from the individual elements (plural secondary light source images) of the fly's eye lens, so that the uniformity of illumination intensity on the reticle can be improved.
In the conventional projection exposure apparatus, the distribution of the illuminating light beam entering the entrance face of the optical integrator such as the above-mentioned fly's eye lens is made substantially uniform (but not completely uniform in practice) in a substantially circular (or rectangular) area around the optical axis of the illuminating optical system.
FIG. 1 schematically illustrates the configuration from the optical integrator to the wafer, in the above-mentioned conventional projection exposure apparatus. An illuminating light beam L130 illuminates a reticle pattern 17 of a reticle R, through a fly's eye lens 11, a spatial filter 12 and a condenser lens 15 in the illuminating optical system. Said spatial filter 12 is positioned at a reticle-side focal plane 11b of the fly's eye lens 11, namely the Fourier transformation plane (hereinafter called pupil plane) to the pattern bearing face of the reticle R, or in the vicinity thereof, and has a substantially circular (or rectangular) aperture around the optical axis AX of the projection optical system, thereby limiting the secondary light source (planar light source) image, formed on the pupil plane, in a circular (or rectangular) area. In this state, the ratio, or so-called .sigma. value, of the numerical aperture defined by the illuminating optical system 11, 12, 15 and the reticle-side numerical aperture of the projection optical system 18 is determined by the diaphragm aperture (for example the aperture of the spatial filter 12), and is generally in a range of 0.3 to 0.6.
The illuminating light beam L130 is diffracted by the pattern 17 formed on the reticle R, whereby the pattern 17 generates a 0-th order diffracted light D.sub.0, a +1st-order diffracted light D.sub.p and a -1st-order diffracted light D.sub.m, which are condensed by the projection optical system 18 and generate, on a wafer 20, complex interference fringes corresponding to the form of the pattern 17. Said interference fringes constitute the projected image of the pattern 17. In this state the angle .theta. (at the reticle side) between the 0-th order diffracted light D.sub.0 and the .+-.1st order diffracted lights D.sub.p, D.sub.m is defined by sin.theta.=.lambda./P, wherein .lambda. is the exposure wavelength and P is the pattern pitch. Solid lines representing the light beam L130 or the 0-th order diffracted light D.sub.0 represent the principal ray emerging from a point on the fly's eye lens 11 or a point on the reticle pattern 17.
As the pattern pitch becomes finer (smaller), sin.theta. becomes larger, and, when sin.theta. exceeds the reticle-side numerial aperture (NA.sub.R) of the projection optical system, the .+-.1st-order diffracted lights D.sub.p, D.sub.m become unable to pass through said projection optical system. In such state the 0-th order diffracted light D.sub.0 alone reaches the wafer W, so that the interference fringes are not generated. Thus, in case of sin.theta.&gt;NA.sub.R, the image of the pattern 17 cannot be obtained, so that the pattern 17 cannot be transferred onto the wafer W.
Based on these facts, there stands a relationship sin.theta.=.lambda./P.apprxeq.NA.sub.R in the conventional exposure apparatus, so that the pitch P is given by: EQU P.apprxeq..lambda./NA.sub.R ( 1)
In a 1:1 line-and-space pattern, the minimum pattern size (width), being equal to a half of the pitch P, is about 0.5.multidot..lambda./NA.sub.R. In the practial lithography, however, a certain depth of focus is required because of wafer curvature, influence of steps on the wafer resulting from the process, or thickness of photoresist itself. Consequently, the practical minimum resolved pattern size can be represented as k.multidot..lambda./NA, wherein k is so-called process coefficient and is in a range of about 0.6 to 0.8. Since the ratio of the reticle-side numerical aperture NA.sub.R to the wafer-side numerical aperture NA.sub.W of the projection optical system is equal to the projection magnification of said system, the minimum resolved pattern size on the reticle is k.multidot..lambda./NA.sub.R, and the minimum pattern size on the wafer is k.multidot..lambda./NA.sub.W =k.multidot..lambda./M.multidot.NA.sub.R wherein M is the projection magnification (reduction rate).
Therefore, for transferring a finer pattern, it has been necessary either to adopt an exposure light source emitting the light of a shorter wavelength, or to employ a projection optical system with a larger numerical aperture. It is naturally conceivable also to optimize both the wavelength and the numerical aperture. Also so-called phase shift reticle, for shifting the phase of the light transmitted through particular portions in the transparent areas of the reticle pattern, by .pi. with respect to that of the light transmitted through other portions, has been proposed for example in the Japanese Patent Publication No. 62-50811. Said phase shift reticle enables transfer of finer pattern than in the conventional art.
Also there has been proposed an inclined illumination method, based on the illumination of the reticle with light inclined by a predetermined angle. Said inclined illumination method is equivalent, in the basic principle, to a method for limiting the form of the secondary light source plane at a plane corresponding to the Fourier transformation of the reticle pattern or in the vicinity of said plane (hereinafter called "modified light source method"), reported at the Fall 1991 Convention of Applied Physics etc.
The present inventors already disclosed a unique system of the inclined illumination method, in the U.S. patent application Ser. No. 791,138, filed Nov. 13, 1991.
Also the idea of applying an inclined illumination method to the projection exposure apparatus was disclosed in the U.S. Pat. No. 4,947,413. The projection exposure apparatus disclosed therein improves the resolution by further providing a spatial filter, capable of transmitting the diffracted light of a selected order, at the Fourier transformation plane (pupil plane) in the projection optical system.
The invention disclosed in said U.S. Pat. No. 4,947,413 can be considered substantially same as the inclined illumination technology disclosed in the Japanese Patent Laid-open Application No. 61-41150, which was already publicly known at the filing of said U.S. Patent.
In the conventional exposure apparatus, however, it is presently difficult to adopt an illumination light source of a wavelength shorter than the present one (for example less than 200 nm), for example because of absence of the optical material suitable for use as a transmissive optical member.
Also the numerical aperture of the projection optical system is already close to the theoretical limit, and a further increase is hardly realizable. Even if a further increase in the numerical aperture is possible, the depth of focus represented by .+-..lambda./2NA.sup.2 decreases rapidly with the increase in numerical aperture, so that the practically needed depth of focus becomes less.
On the other hand, the phase shift reticle has a high cost because of complicated manufacturing process, and still involves various problems such as the unestablished methods for inspection and correction.
Furthermore the modified light source method is associated with the drawbacks of light amount loss or deteriorated uniformity in the illumination intensity, because the form of the secondary light source is limited by a shield plate (spatial filter) with a predetermined aperture, positioned at the plane corresponding to the Fourier transformation of the pattern bearing face of the reticle or in the vicinity thereof (particularly at the exit end of the fly's eye lens).